Injection molding nozzles are well known and are used to inject materials, such as plastic, into the cavity of a mold. For example, such nozzles receive molten material, such as plastic, from an injection molding machine and direct the same into a mold cavity through a passage called a gate. When an injection operation is complete, and prior to opening the mold cavity to eject the molded part, the transfer of molten material through the gate must be stopped. Generally, two methods exist for stopping the transfer of molten material through the gate, namely: thermal, or open, gating; and valve gating.
In thermal gating, the gate is an open aperture through which molten material passes during an injection operation. The gate is rapidly cooled at the end of the injection portion of the cycle, when the injection pressure is removed, to "freeze" the injected material into a plug. This plug remains in the gate to prevent drool of molten material from the gate when the mold is open for the ejection of the molded part. In the next injection portion of the cycle, the cooling applied to the gate is removed and hot molten material from the injection molding machine pushes the plug into the mold cavity, where it melts and mixes with the newly provided molten material.
In valve gating, the opening and closing of the gate is independent of injection pressure and/or cooling and is achieved mechanically with a valve stem. This stem can be moved between an open position, wherein flow of molten materials through the gate is permitted, and a closed position wherein the gate is closed by entry of the valve stem into the gate which establishes a seal, preventing molten materials from passing through the gate. Valve gating is well known and examples of such systems are shown in U.S. Pat. Nos. 2,878,515; 3,023,458; and 3,530,539.
Generally, valve gating is preferable to thermal gating because it can reduce the undesired gate vestige which results on the finished molded part. However, there are problems with valve gating systems.
Specifically, the valve stem and gate each have a complementary sealing portion, usually tapered, which are brought into contact to seal the gate. As the sealing portion of the stem of the valve gate is moved into contact with the sealing portion of the gate, a thin film layer of molten material can be trapped between the sealing portions of the gate and the stem. This thin film layer can prevent the tip of the valve stem from fully extending into the gate and, consequently, the molded part can have an unacceptable gate vestige, in the form of a projection or "crown" in the gate area. Generally, the larger the sealing portions of the stem and gate, the more problematic the thin film layer becomes as more material must be expressed out of the contact area of the sealing surfaces and must be moved further to exit the contact area, making it more likely that some material will remain as a thin film layer. Thus, it is desired to reduce the contact area between the stem and gate by having small sealing portions.
Also, as the tip of the stem is in the flow of molten material when the gate is open, it can become quite hot. When the gate is closed by the stem, the hot tip of the stem can be difficult to cool as the mold cavity is cooled and this can result in a need for increased cycle times to permit the necessary cooling, and/or can result in undesirable characteristics in the molded part. Specifically, as the material in the mold cavity adjacent the stem tip is cooled less efficiently due to the hot tip, parts molded from thermally sensitive materials such as PET can suffer from crystalinity or other undesired characteristics.
Cooling the stem tip is subject to two problems. First, the above-mentioned thin layer can prevent cooling of the stem tip, by acting as an insulator between the stem tip and the cooled gate, and can aggravate the problem of tip cooling and its related undesired consequences. Second, as mentioned above, in order to reduce the size and/or probability of presence of a thin film layer and the gate vestige which can result therefrom, the sealing portions of the gate and the valve stem are usually kept small. However, the resulting relatively small contact area between the stem tip and the gate reduces the rate of thermal transfer of heat from the stem tip to the cooled gate.